1. Field of the Invention
The present invention relates to a gas cooled current lead for a superconducting machine.
A superconducting machine operated using liquid helium has a tendency to grow larger increasingly and for its electric current to increase in strength. Such a machine requires a current lead for carrying an electric current from room temperature to liquid helium temperature. The current lead is a significant source of heat leakage into the liquid helium temperature zone. The flow rate of helium gas cooling in the current lead is often required to be larger than the designed value for passing the necessary magnitude of electric current for the reason that the efficiency of heat exchange within the current lead is not as designed. In such a situation, there is a danger that the balance of the refrigeration system will be lost, thereby permitting the superconducting machine to fall into an inoperable state, since the helium gas which ought to be returned to a helium liquifier/refrigerator cooling the superconducting machine is instead used for the current lead. Such a state has hitherto often come about due to this problem.
A problem during manufacture of current leads is dependent on whether a structure having the same efficiency of heat exchange as designed and a manufacture thereof are possible or not. Generally, a current lead is composed of a conductive material capable of carrying a current inserted into a tube made of stainless steel, through which a helium gas passes. However, the current lead heretofore in use has been made as shown in FIGS. 1(a) and 2(b) because it is difficult to insert a conductive material into a long tube.
FIG. 1(a) shows a tube 1 made of stainless steel. A helical plastic spacer 3 and an electric conduit 2 are inserted in the tube 1. A cold helium gas runs helically between the plastic spacer 3 and the electric conduit 2.
The inside of the electric conduit 2 is made hollow. With this type of current lead, the efficiency of heat exchange is low, since the contacting area between the electric conduit 2 and the helium gas is small.
FIG. 1(b) shows a tube 1 made of stainless steel. A number of fine electric conductor wires 4 are inserted in the tube 1. With this type of current lead, although the area of the fine electric conductor wires 4 contacting the helium gas is large, the percentage of the cross-sectional area of the inside of the tube 1 occupied by the fine electric conductor wires 4 is small, and the helium gas runs in a larger cross-sectional area. As a result, the efficiency of heat exchange is also low with this type of current lead.